Conventional photovoltaic arrays may be deployed on a flexible or rigid panel or sheet. In some embodiments, the deployed photovoltaic array is substantially planar. Individual solar cells, frequently with a rectangular or generally square-shape and sometimes with cropped corners, are connected in electrical series to form a string of solar cells, in which the number of solar cells used in the string determines the output voltage. Individual solar cells, or strings of solar cells, can also be interconnected in parallel, so as to increase the output current of the array. The individual solar cells are provided with electrical interconnects that facilitate the series or parallel connection of cells to form an array. The solar cells are configured to collect the energy or power of the solar radiation and output the energy as electricity in direct current (DC) form. The solar radiation received may be both from the incident light coming directly from the sun and from the diffused light resulting from intermediate reflections.
The overall performance of the photovoltaic arrays relies on the electrical requirements (current-voltage, i.e. I-V), namely: current that varies with total cell area, level of incident solar intensity, the panel photon-current conversion characteristics that degrade over time; and voltage that varies with the number of cells in series, the operating temperature, and the photon-current panel conversion characteristics that degrade over time.
For vehicles operating in space, acceptable performance often appears achievable at Beginning-of-Life (BOL), even for designs providing enhanced shielding to the solar cell for higher benefits at End-of-Life (EOL). But missions go through a variety of complex environments, including the vastly different illumination and temperatures seen during normal earth orbiting missions, as well as for deep space missions, operating at different distances from the sun, such as at 0.7, 1.0, and 3.0 AU (AU meaning astronomical units). The photovoltaic arrays must also endure anomalous events from space environmental conditions, and unforeseen environmental interactions during exploration missions. Hence, radiation exposure, collisions with space debris, and/or normal aging in the photovoltaic array and other systems could cause suboptimal operating conditions that degrade the overall power system performance, and may result in failures of one or more solar cell arrays that, in an uncontrolled array of high voltage solar cell strings, may cascade to other arrays of solar cells.
The power produced by the photovoltaic array depends on the incident light intensity, and is transmitted by power distribution lines at a voltage that is a compromise between: the maximum voltage at which the solar cell array can safely operate, and higher voltages desired for efficient transmission.
Vehicles need photovoltaic arrays to deliver high voltage power for supplying high power generation systems: in order to minimize the mass of the power distribution lines or electrical power harness, and for applications that need 300V or greater, such as direct drive solar electric propulsion (SEP), for directly driving the SEP power processing unit.
Solar cell strings face challenges achieving high voltage: a high numbers of solar cells in series with each string complicates cell layout; environmental interactions especially with respect to electrostatic discharge (ESD) happen as voltage exceeds 33V; and high voltages exacerbate untoward interactions with the natural space plasma environment in earth orbit.
The aforementioned low efficiency in transmission at low voltage is because the major penalty of high current is resistive losses. Hence, power loss and the resulting loss of efficiency increases very rapidly with current (being proportional to current squared times resistance); and therefore, the efficiency is enhanced by lowering current and increasing the voltage as much as possible until approaching voltage breakdown of materials.
State-of-art photovoltaic arrays are typically passive electronics, i.e. they have no power management and control on the array associated with a single panel. Without voltage changing capability on the photovoltaic array, solar cell strings need to deliver their power at a voltage that can be safely operated, which then imposes a low voltage requirement on the solar array's power collection harness. The solar cell string voltages are further limited because of even higher voltages in other operating environments and phases of the mission. For vehicles operating in space, worst case conditions may include the highest temperatures seen in a particular part of each orbit or phase of the mission and/or degradation conditions associated with harsh space environments. The voltages can be very high on the same mission, lowest temperatures for which solar cells have possibly undesirable increased voltage, for example, during a phase of the mission that has a cold environment at 3 AU or during a part of the orbit that is post-eclipse. Designing and operating these solar cell strings at high voltage increases the interactions of the strings with space plasma environment.
Thus, the voltage and current characteristics of the delivered power are normally conditioned and regulated to that needed by the loads in a decentralized manner inside the vehicle, for example, using either a power management unit that is located inside the vehicle architecture as a unique and common point connected by a power bus lines with the loads, or located within the load itself.
By providing a single, unique, and common point to condition and regulate the power delivered to the loads, the overall performance of the system is fixed and unchanged. Moreover, the photovoltaic array may be sized to match within a narrow operational window of electrical current requirements; having therefore no option to change the performance of the system for adaption to the operational environment. Also, the power is transferred through the interface without being regulated, which can result in sub-optimal design and operational conditions for the interface components.
Additionally, if this centralized power management unit unexpectedly fails, there is no intermediate point to assume its function, and therefore, the voltage of the power produced is directly transmitted to all the loads and may result in a malfunctioning of the system.